World Landslide Forum

The simple idea that a landslide could occur is frightening people in every area prone to such phenomena. That is because the effects of landslides can be devastating, leaving thousands of people without a home, threatening their lives and those of their loved ones.

Landslides are usually caused by several factors, but three of them are preeminently important:

Slope saturation with water;

Seismic activity in the area;

Volcanic activity.

Water and Landslides

Water is a primary cause for landslides, as, when present in large quantities, it saturates the superficial slope soil layers and causes them to slide down. Intense rainfall or melting snow, changes occurred in ground-water levels or changes of water levels along coastlines, lake banks, earth dams, canals, reservoirs, and rivers – all these may cause mass movements leading to landslides.

There is always a relationship between flooding and landslides, because both are related to water saturation of the ground. Moreover, mud flows and debris flows occurring in steep stream channels can be mistaken for floods, but it is a fact that these two phenomena often occur at the same time and in the same area.

Landslides may cause flooding when they form landslide dams that eventually block stream channels and valleys and allow large masses of water to back up. Backwater starts flooding and, when the dam fails, it will cause subsequent downstream flooding. Solid debris can add both volume and density to a normal stream flow and can cause channel blockages. Another effect of landslides is overtopping reservoirs and reducing their capacity to store water.

Seismic Activity and Landslides

Certain mountainous areas are vulnerable to landslides but have also been affected by earthquakes, at least at moderate rates. When an earthquake occurs in such areas, the risks for a landslide to occur as well grow tremendously. Ground shaking allows water to infiltrate rapidly between ground layers and then make these layers slide on one another. Ground shaking also causes widespread rock falls.

Volcanic Activity and Landslides

Volcanic activity is the factor that provokes the most devastating type of landslides. Lava melts snow at high speed and can cause a deluge of soil, rock, ash, and water which rushes down on the steep slopes of the volcano. This deluge devastates everything in its path. Volcanic debris flows at great distances and can damage the structures in areas surrounding the volcano. There were numerous cases when the eruption of a volcano triggered a massive landslide.

The inhabitants of every state in the United States know what a landslide is, because such phenomena occur quite frequently in various parts of the fifty states. It is true that the mountainous and coastal areas of Oregon, California and Washington are the most affected regions, but that does not mean that the other areas are all safe.

In the United States, landslides cause about $3.5 billion in damage annually. They also kill from 25 to 50 persons each year. Casualties are caused by rock slides, rock falls and debris falls. In order to know this phenomenon better, and eventually protect themselves from its destructive action, people should be aware of how landslides are formed and how they act.

Types of Landslides

The word “landslide” describes a multitude of processes that have as a result downward and outward movements of materials like:

Soil;

Rock;

Artificial fill;

A combination of the above materials.

When these materials start moving, they may be:

Falling;

Sliding;

Toppling;

Spreading;

Flowing.

The Anatomy of Landslides

Usually, landslides are associated with mountainous areas, but they often affect low relief areas. The factors that could trigger a landslide in a low relief area are:

Cut-and-fill failures determined by building or roadway excavations;

Collapse of mine piles, mainly coal;

Slope failures associated with open-pit mines and quarries;

Lateral spreading landslides;

River bluff failures.

Many kinds of mass movements are referred to as “landslides”, but there is a more restrictive use of this term. Its restrictive meaning refers to mass movements which have a clear zone of weakness, which separates the sliding material from the more stable, underlying one.

From the point of view of their movement, landslides are either rotational or translational, according to the trajectory they follow.

Can Landslides Be Avoided and How?

It is hard to say if landslides can be stopped, but, depending on the location and type of human activity, their effect could be lessened. Hazard zones exist, and people should know them and avoid activities like digging in such areas.

It is the responsibility of the local governments to establish rules meant to reduce the effects of eventual landslides. Land-use regulations and policies are required in areas that are prone to landslides. The absence of such regulations and destructive human activities are among the main factors that favor a landslide.

Whenever a landslide occurs, no matter if it is caused by slope saturation with water, seismic activity or a volcanic eruption, the damages are disastrous. Thousands of households may be swept away or buried in mud and tens to hundreds of people could lose their lives.

This apocalyptic image should make local governments pay more attention to the prevention of such natural phenomena. It is important for a local government to know which areas are prone to landslides and take appropriate measures in order to reduce vulnerability to such hazards.

Can the Effects of Landslides Be Reduced?

Vulnerability to landslides depends on location, frequency of landslide events and type of human activity in the area, but there are also other factors that may influence the size and frequency of such phenomena.

The effects on people and buildings can be lessened if hazardous areas are avoided or if activities in such areas are restricted or deployed under certain conditions. Local governments are responsible for land-use policies and other regulations meant to reduce the risks for landslides to take place.

Exposure to hazards may be reduced if individuals educate themselves on the past history of these phenomena. Departments of local governments that are responsible with planning and engineering may help a lot with their advice.

People can also benefit from the professional services of engineering geologists, civil engineers, or geotechnical engineers, all qualified to evaluate the potential of a hazardous site, no matter if built or not built.

Four Actions Meant to Increase Ground Stability and Prevent a Landslide

Landslides hazard can be reduced by avoiding to emplace new constructions on steep slopes or by stabilizing the slopes before beginning the actual construction works. When ground water cannot rise in the landslide mass, stability is increased. To achieve this result, some actions are required:

Covering the land with impermeable membranes in order to prevent water infiltration in the landslide;

Directing surface water sources away from the landslides;

Draining ground water streams away from the landslides;

Minimizing irrigation on the surface of the soil.

To achieve increased slope stability, it is also useful to remove mass from the top of the slope, so that its weight does not force the layer to slide.

Due to the huge losses that landslides imply, their prevention is of maximum importance for all the people living in the area of hazard. Preventing a landslide from causing material damage and human losses should be a main goal of local authorities.